Add end-to-end muxed-contract storage-key test; dedupe with account case

Extract the shared body of the muxed-account storage-key test into a
helper `check_muxed_address_is_not_allowed_as_storage_key`, and add the
CAP-0084 muxed-contract twin as a thin, feature-gated wrapper. Also
record the missing observation for the `muxed_contract_storage_key_conversion`
unit test.

Author: sisuresh

soroban-env-host/observations/test__storage__muxed_contract_storage_key_conversion.json

@@ -0,0 +1,4 @@
+{
+  "   0 begin": "cpu:14488, mem:0, prngs:-/9b4a753, objs:-/-, vm:-/-, evt:-, store:-/-, foot:-, stk:-, auth:-/-",
+  "   1 end": "cpu:15100, mem:80, prngs:-/9b4a753, objs:-/1@d3ad1bcd, vm:-/-, evt:-, store:-/-, foot:-, stk:-, auth:-/-"
+}

soroban-env-host/src/test/storage.rs

@@ -450,14 +450,14 @@ fn muxed_contract_storage_key_conversion() {
     assert!(res_non_storage.is_ok());
 }
 
-#[test]
-fn test_muxed_account_is_not_allowed_as_storage_key() {
+// Drives an end-to-end contract call for every storage type to assert that
+// `muxed_address` is rejected as a storage key (directly, via `put`/`get`)
+// while still being usable as a stored value. Shared by the CAP-67
+// (muxed account) and CAP-0084 (muxed contract) cases, which differ only in
+// the address they feed in.
+fn check_muxed_address_is_not_allowed_as_storage_key(muxed_address: MuxedScAddress) {
     let host = Host::test_host_with_recording_footprint();
     let contract_id = host.register_test_contract_wasm(CONTRACT_STORAGE_WITH_VALS);
-    let muxed_address = MuxedScAddress(ScAddress::MuxedAccount(MuxedEd25519Account {
-        id: 123,
-        ed25519: Uint256([10; 32]),
-    }));
     let muxed_address_val = host.add_host_object(muxed_address.clone()).unwrap();
 
     let run_test = |storage: &str| {
@@ -512,6 +512,30 @@ fn test_muxed_account_is_not_allowed_as_storage_key() {
     run_test("instance");
 }
 
+#[test]
+fn test_muxed_account_is_not_allowed_as_storage_key() {
+    check_muxed_address_is_not_allowed_as_storage_key(MuxedScAddress(ScAddress::MuxedAccount(
+        MuxedEd25519Account {
+            id: 123,
+            ed25519: Uint256([10; 32]),
+        },
+    )));
+}
+
+// CAP-0084: the muxed contract twin of
+// `test_muxed_account_is_not_allowed_as_storage_key`.
+#[cfg(feature = "cap_0084_muxed_contract")]
+#[test]
+fn test_muxed_contract_is_not_allowed_as_storage_key() {
+    use crate::xdr::MuxedContract;
+    check_muxed_address_is_not_allowed_as_storage_key(MuxedScAddress(ScAddress::MuxedContract(
+        MuxedContract {
+            id: 123,
+            contract_id: ContractId([10; 32].into()),
+        },
+    )));
+}
+
 #[test]
 fn test_storage_mix() {
     // This makes sure the keyspaces are not mixed between storage types.